1. Field of the Invention
This invention relates generally to the field of integrated circuit design and fabrication. Specifically, the invention relates to methods for making local interconnect structures for integrated circuit devices and the structures formed thereby.
2. State of the Art
Integrated circuits (ICs) contain individual devices which are interconnected during fabrication by an intricate network of conductive material. The quality of these interdevices interconnections in part determines the performance and reliability of the overall IC device.
Local interconnects, unlike other interconnects such as multi-level interconnects, electrically connect individual devices of the overall IC device at a level or levels below customary metallization levels. For example, local interconnects connect gates and emitters to diffusion areas and N+ and P+ regions across field oxide regions, thereby increasing the packing density. See T. Tang et al., Titanium Nitride Local Interconnect Technology for VLSI, IEEE Trans. Electron Devices, Vol. ED-34, 3 (1987) p. 682, incorporated herein by reference.
Numerous materials have been employed in local interconnects. These local interconnect materials include titanium nitride and refractory metals, as well as titanium silicide (TiSix). TiSix has been used as a local interconnect material because of its low resistance and high conductivity. However, the quality of local interconnects formed from these materials varies.
Several techniques for fabricating TiSi, local interconnects are described in U.S. Pat. Nos. 4,975,756, 5,124,280, and 5,173,450, all incorporated herein by reference. Another technique of fabricating TiSix local interconnects is disclosed in U.S. Pat. No. 5,654,575, incorporated herein by reference. This latter patent describes titanium silicide interconnect straps which are formed from a silicon-on-titanium bilayer that has been deposited over a titanium nitride (TiN) layer. The TiN layer is formed by depositing a titanium (Ti) layer and then annealing in a nitrogen ambient, thereby nitriding the whole Ti layer. No Ti is left underlying the TiN layer.
Using titanium silicide as a local interconnect material can result in several problems, as explained in U.S. Pat. No. 5,341,016, incorporated herein by reference. One problem is that titanium silicide severely agglomerates when exposed to high temperatures such as 850xc2x0 C. Agglomeration can increase both silicided source/drain and polycide sheet resistances and lead to excessive leakage and/or gate oxide degradation. Another problem with titanium silicide is diffusion of the titanium material into underlying silicon regions during heating, which allows titanium-rich TiSix to consume active areas in the silicon.
A particular problem with titanium silicide has been cratering exhibited during local interconnect formation. When Ti and silicon (Si) are sequentially deposited in a via or hole (such as a trench formed between gate electrodes) and reacted to form an interconnect, sufficient amounts of Si do not migrate to the bottom of the trench to react with the Ti located there. Because of the lack of overlying Si volume at the bottom of the trench, the Ti instead reacts with underlying regions of the silicon substrate and causes craters in these regions. This cratering can reduce the performance of the IC device by increasing the current leakage.
The present invention relates to a method for making a local interconnect structure by forming a Ti layer having a nitrogen-rich upper portion over a portion of a substrate, forming a refractory metal layer on the Ti layer, forming a Si layer on the refractory metal layer, removing a portion of the Si layer, and then heating to form a structure. The nitrogen-rich Ti layer may be formed by depositing Ti in a nitrogen atmosphere, or by depositing a Ti layer and either annealing in a nitrogen-containing atmosphere or ion implanting nitrogen.
The present invention also relates to a source structure for a local interconnect. The source structure comprises a Ti layer having a nitrogen-rich upper portion overlying a portion of a substrate, a refractory metal layer overlying the Ti layer, and a silicon layer overlying the refractory metal layer. The lower portion of the Ti layer preferably contains substantially no nitrogen. The nitrogen-rich upper portion of the Ti layer preferably extends along the upper surface of the Ti layer. The refractory metal is preferably Ti and the silicon layer is preferably amorphous silicon.
The present invention also relates to a local interconnect structure. The local interconnect structure comprises a titanium silicide layer disposed over a portion of a substrate, a nitrogen-rich Ti layer disposed on the titanium silicide layer, and a refractory metal silicide layer disposed on the nitrogen-rich Ti layer. The titanium silicide layer is preferably disposed over active areas of the substrate. The refractory metal is preferably Ti.
The present invention produces several advantages when compared to the prior art. The present invention reduces cratering and consumption of Si regions underlying the local interconnect structure because unconsumed Ti is not available to react with the silicon regions during later heat treatments. Subsequent processing, therefore, can proceed at higher temperatures. Another advantage is that less current will leak to the substrate because smaller craters in the underlying silicon substrate permit less current to leak. The present invention also forms a more continuous silicide interconnect because greater amounts of the Ti react with Si to form the local interconnect. The more continuous local interconnect, in turn, produces less break-through of overlying metal layers which contact the local interconnect.